Nitroxyl - from a nitric oxide-related derivative to an independent signaling molecule in plants.

Nitric oxide (•NO) is a central regulator of plant development and stress responses, but recent evidence shows that reactive nitrogen signaling involves more than •NO alone. Nitroxyl (HNO), the one-electron-reduced and protonated form of •NO, has emerged as a distinct signaling molecule in plants with unique chemical and functional properties. Unlike •NO, HNO is a non-radical, thiophilic species that acts as a selective redox modulator rather than a freely diffusible messenger. Improved detection methods have confirmed endogenous HNO formation in plant cells, with basal nanomolar concentrations under optimal conditions and reversible changes during key developmental stages and stress responses, such as senescence and hypoxia. HNO production and stability are closely regulated by intracellular redox status and interconvert dynamically with •NO through non-enzymatic reactions involving cellular reductants such as ascorbate, hydrogen sulfide, and low-molecular-weight thiols. This interplay establishes the •NO/HNO balance as a redox-sensitive signaling switch, increasing the informational capacity of reactive nitrogen networks. Functional and transcriptomic studies indicate that HNO participates in hormone signaling, especially in ethylene-mediated pathways, and supports plant adaptation to redox-related stresses. This review summarizes current knowledge of HNO chemistry, regulation, detection, and function in plants, and identifies key open questions. Including HNO in plant signaling frameworks provides a more nuanced and comprehensive view of redox-regulated plant physiology.